power meter drift issues?

hey all,

unfortunately i've fallen into the power meter accuracy snake pit.

recently i've been seeing what looks to me like power meter drift. you
can see a pretty good example of this from my last trainerroad workout
here (done on a pre-2017 wahoo kickr snap):

https://analyze.dcrainmaker.com/#/public/ade655fb-936b-462b-6037-ef06a2775681

one power meter is a set of garmin vector 3 pedals with a claimed
accuracy of +/- 1.0%. (i've followed the service advisory notes for the
vector and verified the spindle torque. after installing the vectors i
reset the install angles and i also did a static torque test, as
described on the official garmin support pages, using a rice lake
calibrated 20 kg weight). the second power meter is a powertap g3 hub
with a claimed accuracy of +/- 1.5%. both power meters were calibrated
(zero'd) before the workout, which was done indoors at a consistent
temperature of around 70 F.

at the start of the ride, the powertap numbers are a few watts below the
vector numbers, which makes sense given some drivetrain power loss. but
by the end of the ride the power meters have reversed, with the powertap
giving numbers about 10% above the vector numbers, which seems wrong.
(aside from the drift, the difference between the two power values seems
out of spec for claimed accuracy of the devices, especially when you
consider that there should be some drivetrain power losses).

has anyone else seen something like this before? to me it seems like
one of the power meters is drifting out of spec? any ideas what else
might be going on or how to root cause the problem (short of buying
a tacx neo or a third power meter ;)?

cheers,
ed
Parents
  • I did a static weight test after a ride and the scaling factors didn't change much (0.003%)
    Left: Pre-ride: 0.9781; Post-ride: 0.9809; Difference: -0.0028
    Right: Pre-ride: 0.9928; Post-ride 0.9905; Difference: 0.0023

    I disabled auto-zero and did a couple more indoor rides. I determined that doing a manual zero after a bit of warm up changes the values reported for both power meters. But past that I''m not able to draw any satisfying conclusions from the data and I still see my PowerTap consistently reporting notably higher values than my Vectors (well past the accuracy of both power meters added together), which just seems wrong.


    The relevant take-away here is that you are getting the expected result (within tolerance) from your static weight test on the pedals, and it's not changing through the ride. You can do a similar test on the Powertap hub, but I am not sure what specific tool (i.e. how to configure a head unit) is currently required to do that. I don't know how you validate the Kickr.

    I recalibrated the Vectors after the 1st set (@ ~17:00:32), which resulted in an ~10 W drop in the PowerTap numbers. Since PowerMatch was enabled, this indicates that there was actually a ~10W jump in the numbers being reported by the Vectors.

    Can you post the .fit files for the pedals from these tests? Or email them to [email][email protected][/email]. It may be possible to see exactly what is changing when you did these calibrations.
    I would not expect a large jump after unless something was affecting the pedal installation even after you had put in several rides. Possibilities would include severe undertorquing but also debris between the pedal spindle flange and the crank mounting face or the threads.
Reply
  • I did a static weight test after a ride and the scaling factors didn't change much (0.003%)
    Left: Pre-ride: 0.9781; Post-ride: 0.9809; Difference: -0.0028
    Right: Pre-ride: 0.9928; Post-ride 0.9905; Difference: 0.0023

    I disabled auto-zero and did a couple more indoor rides. I determined that doing a manual zero after a bit of warm up changes the values reported for both power meters. But past that I''m not able to draw any satisfying conclusions from the data and I still see my PowerTap consistently reporting notably higher values than my Vectors (well past the accuracy of both power meters added together), which just seems wrong.


    The relevant take-away here is that you are getting the expected result (within tolerance) from your static weight test on the pedals, and it's not changing through the ride. You can do a similar test on the Powertap hub, but I am not sure what specific tool (i.e. how to configure a head unit) is currently required to do that. I don't know how you validate the Kickr.

    I recalibrated the Vectors after the 1st set (@ ~17:00:32), which resulted in an ~10 W drop in the PowerTap numbers. Since PowerMatch was enabled, this indicates that there was actually a ~10W jump in the numbers being reported by the Vectors.

    Can you post the .fit files for the pedals from these tests? Or email them to [email][email protected][/email]. It may be possible to see exactly what is changing when you did these calibrations.
    I would not expect a large jump after unless something was affecting the pedal installation even after you had put in several rides. Possibilities would include severe undertorquing but also debris between the pedal spindle flange and the crank mounting face or the threads.
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